I use ssh to connect to a remote site (from ubuntu desktop to debian server). When I first connect a dialog pops up (probably from gnome or seahorse) asking for the passphrase, but from then on it connects with no dialog. That's fine, but I would like it to expire eventually. As it is, even days later, I don't need to give the password, until I log out or reboot or run "ssh-add -D" manually (from the ubuntu desktop). How do I set an expiration?

Edit: using some answers from here as clues, I've figured out (I think) that seahorse and ssh-agent are being started from the /etc/X11/Xsession.d on my ubuntu desktop system. There are files for each (seahorse and ssh-agent). It seems like ssh-agent is the go-to program for what I want to do, since seahorse just puts a GUI face on it, but I can't figure out how to specify arguments to ssh-agent. The 90x11-common_ssh-agent file is not very straightforward... it refers to $OPTIONFILE but I have no idea what that variable is set to or how to find out.